The game lags down to 2 fps when the player looks at a cluster of command blocks. Otherwise 30+fps is the average. Only in 14w04a.
See this bug in action: http://youtu.be/nsUtoAhviyM The command blocks all have testfor, fill, clone and blockdata commands in them, if that's relevant.
I looked at other tickets before making this one, all the others seem to be affected by other versions, unlike this one.
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Please force a crash by pressing F3 + C for 10 seconds while in-game and attach the crash report (minecraft/crash-reports/crash-<DATE>-client.txt) here.

Duplicate of Unresolved - MC-36883
This isn't a duplicate of MC-36883. That bug affects versions other than 14w04a, and seems to be inconsistent when this bug is perfectly consistent. This is most likely a duplicate of MC-45568 (which has been mistakenly 'resolved' as a duplicate). I have discovered the source of the lag is actually the item frames I used which duplicate themselves as their items dozens of times over, but only the item frames on the north side of z=0. Item frames on the south side of z=0 are fine.
I would really like to see this fixed in the next snapshot i enjpy working with command blocks but cant because of this bug.